1. Technical Field
The present invention relates generally to wireless communication systems; and more particularly to a wireless communication system in which multiple forward links to a single mobile unit are managed to maximize network capacity while ensuring robust performance during handoffs to maintain the grade of service as determined by error rates, voice quality and probability of dropped calls among other criterion.
2. Related Art
Wireless communication systems are well known in the art. In a typical cellular based wireless communication system, a mobile switching center (MSC) couples to a plurality of base stations to provide wireless coverage within a service area. Within the service area, each of the base stations provide wireless coverage within a respective cell. The cells may be further divided into sectors about the base station wherein separate coverage is provided within each sector of the cell. In combination therefore, the sectors of the cells and the cells provide wireless coverage throughout the service area. In combination, the MSC and base stations are often referred to as the "network."
Mobile and stationary wireless units operating within the service area establish links with one or more of the base stations via respective sectors and/or cells. The links with the base station(s) facilitate communication between the units and other units operating within the service area as well as between the units and units connected to the wireless communication system via the MSC. In most systems, the public switched telephone network (PSTN) couples to the MSC. Thus, the network allows units operating within the service area to communicate with units connected to the PSTN.
As mobile units move about within the service area, they move from sector to sector and/or from cell to cell. To maintain continuous/uninterrupted communication with the network, the mobile unit must dynamically connect and disconnect wireless links during operation within various sectors and/or cells. When the mobile unit dynamically alters links between the various cells/sectors during an ongoing communication, a handoff occurs. During a typical handoff, simultaneous communications are established between multiple base stations/sectors and the mobile unit. The multiple links are maintained during the handoff when needed and disconnected when they are no longer needed or no longer satisfy specified criteria.
For example, in a code division multiple access (CDMA) system, each mobile unit operating within the service area typically requests and establishes links with multiple base stations. In such CDMA systems, the mobile unit continuously measure pilot signal strengths transmitted from sectors and cells of base stations. When the strength of a pilot signal of a sector not currently communicating with the mobile unit exceeds a threshold (T.sub.-- ADD), the mobile unit sends a pilot signal measurement message (PSMM) to the network via already connected sector(s). Based on the PSMM, the network typically establishes an additional link between the mobile unit and the additional sector or cell if resources permit. Once such link is established, through a sequence of messages that may include an enhanced handoff direction message (EHDM) and a handoff completion message (HCM), the sector or cell transmits forward link communications to the mobile unit. Thus, at any given time, multiple sectors in a handoff are typically transmitting identical data to the mobile unit. The mobile unit receives the data from the multiple sectors and uses maximal ratio combining or other combining techniques to combine the multiple received signals.
However, capacity within a CDMA system is limited to a greatest number of mobile units that can be receiving forward link transmissions from any cell or sector. Thus, during excessive handoff wherein mobile units receive forward link transmissions from multiple base stations, the overall capacity of the system is reduced as is the overhead available for other users. Further, because forward link transmissions intended for a particular mobile unit appears as noise to other mobile units within the system, the additional transmissions from the multiple base stations increases the multiple access noise level in the system.
The capacity in a CDMA system in terms of the average number of users (N) may be defined by the following equation: EQU N=(1-(f.sub.Pilot +f.sub.Page +f.sub.Synch)/(g.sup.2 *f*(hrf)-v)
where:
N=the number of users an average sector can support assuming the above conditions; PA1 f.sub.Pilot =the fraction of total HPA power allocated for the pilot channel; PA1 f.sub.Page =the fraction of total HPA power allocated for the paging channel; PA1 f.sub.Synch =the fraction of total HPA power allocated for the synch channel; PA1 g.sup.2 =the normalized average forward traffic channel gain; PA1 f=ratio of pilot power to total HPA power; PA1 hrf=handoff reduction factor, a calculated value which takes into account the required resources due to different types of handoff; and PA1 v=the voice activity factor. PA1 I*n.sub.(j,k) is defined as follows:
The combination of (g.sup.2 *f) represents the average of the transmit power fraction used per transmitting sector for a mobile in the network. Further, the handoff reduction factor may be further defined by: EQU hrf=n.sub.(1,1) +2*n.sub.(1,2) +3*n.sub.(1,3) + EQU ((2*n.sub.(2,2) +3*n.sub.(2,3) +4*n.sub.(2,4))* v.sub.2)/V+ EQU ((3*n.sub.(3,3) +4*n.sub.(3,4) +5*n.sub.(3,5))* v.sub.3)/V+ EQU ((4*n.sub.(4,4) +5*n.sub.(4,5) +6*n.sub.(4,6) +5*n.sub.(5,5) +6*n.sub.(5,6)) *v.sub.4)/V
where:
I=number of sources of power control bits the system is having to send to the one mobile unit; PA2 n.sub.(j,k) =the percentage of time the one mobile experienced this type of handoff; PA2 j=the number of cells with which the mobile is communicating; and PA2 k=the number of sectors with which the mobile is communicating. PA2 v.sub.X =X cell voice activity factor (i.e. the adjusted voice activity to account for variations in power gain of the power control bit due handoff involving X cells; and PA2 V=0.45; the average Markov voice activity factor.
In operation of a system according to the above equations, when excess handoffs occur, hrf is high which lowers the capacity (N) of the system. In prior systems, hrf was difficult to reduce without increasing the (g.sup.2 *f) because of possible diversity loss from delays associated in adding/dropping sectors from the active set, which impacted capacity. Such was the case due to requirements of the prior systems during handoff. Thus, the overall forward link capacity (N) of the prior systems was limited since g.sup.2 *f increased more than hrf decreased.
In order to optimize transmit power requirements, forward link power control is employed. The forward link transmit power (of all sectors involved in soft handoff with the mobile unit) is incrementally reduced by a pre-determined amount whenever good forward link frame reception is reported by the mobile unit on the reverse link. However, whenever a bad frame is reported on the reverse link by the mobile unit, the forward link transmit power is incrementally increased. The incremental increase is generally much larger than the incremental decrease and the increase/decrease ratio required to meet required frame error rates. Thus, an incremental increase reduces resources available to other mobile units.
A power control algorithm is typically implemented by/at the base station controller (BSC) which resides in the MSC. The algorithm determines the forward link transmit power for a particular mobile on a frame by frame basis and increments and decrements the forward link transmit power as described above. The determined power levels are typically sent to the base stations involved in the handoff with the mobile unit. Each base station then transmits forward link data to the mobile unit at the reported power level. Therefore, if the mobile is in a three way handoff, for example with one sector of one base station and two sectors of another base station, each of the sectors in the handoff will typically transmit information on the forward link to the mobile at the same power level at any instant of time.
During each soft handoff, all cells/sectors involved in the handoff transmit at the full power controlled by the power control algorithm employed on respective forward links. However, the forward link transmissions may not be required by the mobile unit in the handoff and reduce the pool of available resources for other users. Further, the unneeded forward link transmissions increase the multiple access noise floor in the system. Resultantly, the network capacity is reduced.
Thus there exists a need in the art for a wireless communication system in which forward link transmit power is managed so as to maintain/improve the grade of service and increase system capacity.